Generalized adaptive MTI system

ABSTRACT

An adaptive N-pulse MTI processor for obtaining optimum clutter cancellation with a minimum of hardware, comprising a first input pulse line; first delay means for providing an interpulse delay T to pulses on this first input line; a first adaptive canceller with its auxiliary input connected directly to take pulses from the first input line, and with its main input connected to take delayed pulses from the first delay means; and N-2 series-connected canceller circuits, with each canceller circuit associated with a different level of pulses. The nth canceller circuit, where 1≦n≦N-2, includes an nth adaptive canceller; an nth input pulse line; n series connected delay means for providing a series of interpulse delays T to pulses propagating on the nth input pulse line, and with n+1 terminals for taking pulse outputs in front of each of the n delay means and after the last of the n delay means; and nth commutating switch means with a plurality of switch positions connected for applying pulses from the n+1 terminals to the main and auxiliary inputs of the nth adaptive canceller. The commutating switch means operates to provide appropriate sets of two pulses from the n+1 terminals to the main and auxiliary inputs to effect the n+2 canceller level of Gram-Schmidt pulse decorrelation on the output line of the nth canceller. The output from the n=1 adaptive canceller is connected to the first input pulse line. The output from the first adaptive canceller then provides the Gram-Schmidt decorrelation. With this configuration, only N-1 cancellers are required to implement the optimum Gram-Schmidt decorrelation for an N-pulse MTI.

BACKGROUND OF THE INVENTION

The present invention relates generally to improvements in signalprocessing systems, and more particularly to improved techniques anddesigns for eliminating interference or otherwise undesirable signalcomponents from serial data samples to be processed by a moving targetindicator type radar system.

As is well known in the art, desired information received by acommunication, sonar or a radar system is frequently not isolated byitself, but is found in the presence of unwanted signals. These unwantedsignals typically vary much more slowly with time than the desiredsignals. Thus, these unwanted signals generally are correlated from datapulse sample to data pulse sample. With specific respect to a movingtarget indication radar system, radar echos from non or slowly movingradar reflectors such as ground clutter, sea returns, and reflectionsfrom wind driven interference such as rain and chaff, all generate echoswhich have frequency components which are relatively slowly changingwithg respect to a moving target. Thus, the echos from this radarclutter are correlated between successive radar pulse samples, i.e.,these echo components appear the same in adjacent data pulse samples.Accordingly, since rapidly moving reflectors, i.e., targets, do notcorrelate from data sample to data sample, it is possible tosignificantly enhance the moving target signal by removing thecorrelated signal components resulting from sea, ground, rain, and chaffechos.

Prior art MTI systems utilize cancellers in order to decorrelate theoutput of a data pulse sample from an adjacent data sample. The typical2-pulse adaptive canceller operates to phase shift and amplitude weightone of the data samples, and then to subtract this phase shifted andweighted data sample from another of the data samples. Such systems workwell to eliminate correlated interference when only one narrowbandinterference source is present. However, when multiple and/or widebandinterference sources are involved, a multiple degree-of-freedom systemis needed to cancel the correlated components of the data samples.Theoretically, if N independent interference sources are present in asignal environment, the interference signals may be cancelled bymultiple cancellers fed by inputs from separate data sample pulses. Inpractice, however, it has been found that effective cancellation cannotbe obtained unless the data pulse sample inputs are independent, i.e.,decorrelated from one another, in order to prevent the reintroduction ofsignals which have been cancelled in a previous canceller's circuit.

A typical prior art adaptive moving target indicator system forproviding optimum clutter cancellation utilizing independent auxiliarydata samples, requires N(N-1)/2 cancellers, where N is the number ofpulses used in the MTI. A prior art MTI structure of this type is shownin FIG. 1. This configuration utilizes Gram-Schmidt processing usingseries iterative cancellation. FIG. 1 is broken up by means of dashedlines in order to show a 2-pulse canceller configuration, a 3-pulsecanceller configuration, a 4-pulse canceller configuration, and a5-pulse canceller configuration. The extension to higher orderconfigurations is clear. A standard tapped delay line 10 is utilized inorder to obtain multiple data samples. Each delay line provides a delayequal to the interpulse delay T between radar pulss. Accordingly, for a2-pulse cancellation system, a radar pulse return signal is applied fromthe terminal 14 directly to the auxiliary input of canceller 16 andindirectly through an interpulse delay element 12 to the main input ofthe canceller 16. The result of this cancellation for a series of pulsesamples numbered consecutively 1, 2, 3, 4, . . . is obtained on line 18and, for the first two pulses numbered 1 and 2, is equal to 1⊥2=A, i.e.,the components of the pulse sample 1 that are perpendicular to thecomponents of pulse sample 2, or more succinctly stated, pulse sample 1decorrelated from pulse sample 2. The 2-pulse canceller is sufficient tocancel one source of narrowband interference.

In order to cancel additional or wideband sources of interferecne, 3-,4-, and 3-pulse cancellers are utilized. In FIG. 1, a 3-pulse cancelleris formed by adding a second interpulse delay segment 20 in conjunctionwith a second canceller 22 and a third canceller 24. In operation,pulses are applied from the terminal 26 in the delay line 10 directly tothe main input of the canceller 22 and then indirectly through theinterpulse delay element 20 to the auxiliary input of the canceller 22.The resulting output from the canceller 22 is pulse 3 decorrelated withpulse 2 and is represented as 3⊥2=B. B is then applied on the line 30 tothe auxiliary input to the canceller 24. The ouput A on line 18 isapplied to the main input of the canceller 24. The output from thecanceller 24 on line 32 is A decorrelated from B and is represented byA⊥B. In order to obtain a 4-pulse canceller, a third interpulse delayelement 34 is utilized in the delay line 10 and three additionalcancellers 36, 38, and 40 are included. In operation, pulses are takenfrom the terminal 42 in the delay line and are applied directly to themain input of the canceller 36. The signal at point 14 in the delay lineis applied via the line 44 to the auxiliary input to the canceller 36.In the example shown in the figure, pulse 2 in the pulse sequence isapplied to the auxiliary input of the canceller 36 while pulse 4 isapplied directly to the main input of the canceller 36. The result ispulse 4 decorrelated from pulse 2 and is represented by 4⊥2=C on line48. C is then applied to the main input for the canceller 38. B from thecanceller 22 on line 30 is applied to the auxiliary input for thecanceller 38. The resulting output from the canceller 38 is on line 50and is C decorrelated from B and is represented by C⊥B. C⊥B is appliedvia the line 52 to the auxiliary input to the canceller 40. The signalon line 32, A⊥B, is applied to the main input of the canceller 40. Theresulting output from the canceller 40 is on line 54 and is (A⊥B)⊥(C⊥B).

Finally for a 5-pulse canceller, a fourth interpulse delay element 60 isutilized in the delay line in conjunction with the additional cancellers62, 64, 66, 68. The fifth pulse is applied from the terminal 70 directlyto the main input of the canceller 62. Pulse 2 from terminal 14 in thedelay line is applied via the line 71 to the auxiliary input to thecanceller 62. The resulting output from the canceller 62 is found online 72 and is pulse 5 decorrelated from pulse 2 and is represented by5⊥2=D. The output from the canceller 62, i.e., D, is applied to the maininput of the canceller 64. The output B on line 30 from the canceller 22is applied to the auxiliary input of the canceller 64. The resultingoutput from the canceller 64 is found on line 74 and is D decorrelatedfrom B and is represented D⊥B. The output D⊥B is applied to the maininput of the canceller 66. The output C⊥B on line 52 from the canceller38 is applied to the auxiliary input for the canceller 66. The resultingoutput from the canceller 66 is found on line 76 and is (D⊥B)⊥(C⊥B).This output on the line 76 is applied to the auxiliary input to thecanceller 68. The output (A⊥B)⊥(C⊥B) is applied on line 54 to main inputto the canceller 68. The resulting output from the 5-pulse canceller isfound on line 80 and is [(A⊥B)⊥(C⊥B)]⊥[(D⊥B)⊥(C⊥B)].

It can be seen from the above discussion of FIG. 1 that in order toobtain optimum clutter cancellation, a significant number of cancellersare required. In this regard, as the number of pulses in the pulsecancellation system increases, the number of required cancellers rapidlyincreases, i.e., N(N-1)/2 cancellers are required for N pulses. In theexample shown in FIG. 1 for a 5-pulse MTI, 10 cancellers are required.This large number of cancellers required to obtain an optimum cluttercancellation is so costly that it is essentially impractical.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptimum clutter cancellation with a minimum of hardware and cost.

It is a further object of the present invention to significantlysimplify the implementation of an optimum multiple-pulse adaptive MTIprocessor.

Other objects, advantages, and novel features of the present inventionwill become apparent from the detailed description of the invention,which follows the summary.

SUMMARY OF THE INVENTION

Briefly, the present invention comprises an adaptive N pulse MTIprocessor for obtaining optimum clutter cancellation with a minimum ofhardware, comprising a first input pulse line; first delay means forproducing an interpulse delay T to pulses on the first input line; afirst adaptive canceller with a main and auxilliary inputs and an outputline, with the auxiliary input connected directly to take pulses fromthe first input line, and with the main input connected to take delayedpulses from the first delay means, the first adaptive canceller fordecorrelating a signal applied to the main input from a signal appliedto the auxiliary input and generating a residue signal on the outputline thereof; N-2 series connected canceller circuits, with eachcanceller circuit associated with a different number of pulses, with thenth canceller circuit, where 1≦n≦N-2, including an nth adaptivecanceller, with a main and an auxiliary inputs, and an output line, fordecorrelating a signal applied to the main input from a signal appliedto the auxiliary input and generating a residue signal on the outputline; an nth input pulse line; n series connected delay means forproviding a series of interpulse delays T to pulses propagating on thenth input pulse line, and with n+1 terminals for taking pulse outputs infront of each of the n delay means and after the last of the n delaymeans; nth commutating switch means with a plurality of switch positionsconnected for applying pulses from the n+1 terminals to the main andauxiliary inputs of the nth adaptive canceller, with the nth commutatingswitch operating to provide appropriate sets of two pulses from the n+1terminals to the main and auxiliary inputs to effect one canceller levelof Gram-Schmidt pulse decorrelation on the output line of the nthcanceller; wherein the output line for the nth adaptive canceller isconnected to the n-1 input line for the n-1 canceller circuit, andwherein the n=1 canceller circuit has the output line from its adaptivecanceller connected to the first input line, and wherein the output linefor the first adaptive canceller provides the full Gram-Schmidt pulsedecorrelation output.

In a preferred embodiment, the adaptive cancellers utilized in thepresent design are digital adaptive cancellers.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic block diagram of a prior art canonical movingtarget indicator (MTI) structure.

FIG. 2 is a schematic block diagram of one embodiment of a 3-pulse MTIin accordance with the present invention.

FIG. 3 is a schematic block diagram of a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention discloses an adaptive N pulse MTI processor forobtaining optimum clutter cancellation with a minimum of hardware. Thisinvention, for an N-pulse MTI, places N-1 cancellers or pulsedecorrelators in series, and utilizes a plurality of tapped delay linesfor obtaining pulse samples along with a plurality of commutatingswitchs for providing the proper pulses from the tapped delay lines tothe main and auxiliary inputs of the individual cancellers. Inparticular, for each canceller except one, there is a set of at leasttwo switches for providing the proper pulse inputs to that canceller'smain and auxiliary inputs. The logic utilized in the present designpermits optimum clutter cancellation with only N-1 cancellers instead ofN(N-1)/2 cancellers as required in the prior art.

Before discussing the logic of FIG. 2 in detail, a brief discussion willbe provided of the operation of a standard canceller or pulsedecorrelator. The MTI operating assumption is that a moving target willprovide a small echo amplitude and be in just one range cell.Accordingly, there should be no correlation of the target echo pulsebetween adjacent pulse samples. In contrast, ground clutter, waves,chaff, and wind driven interference provide echos which change veryslowly in amplitude and phase relative to the moving target echos.Accordingly, these clutter echos will correlate from pulse to pulse. Thepurpose of the canceller is to subtract various sets of pulses in orderto reduce this correlated clutter to zero. The basic equation for thecanceller is R=M-WA, where R is the residue remaining after thecancellation process, M is the signal applied to the main input of thecanceller, A is the signal applied to the auxiliary input of thecanceller, and W is the weight which is to be multiplied with theauxiliary signal A in order to cause WA to approach the value of themain signal M and reduce R to zero. The weight W is obtained byaveraging over a series of n range cells and is set up primarily on theclutter echos. Typically, it is desired to minimize the power of theresidue R in the least squares sense. This minimization is accomplishedvia the orthogonality principal, i.e., that R should be orthogonal toA*, the complex conjugate of A. This orthogonality results in theequation RA*=0, which must be averaged because this is a random process.The weight W can be obtained by first taking the standard cancellerequation R=M-WA, then multiplying this equation by A* to yield thefollowing equation RA*=MA*-W AA*. This equation is then solved for W toyield the following equation: W=MA*/AA*. In essence, the weight Wchanges the amplitude and the phase of the auxiliary signal A to makethe residue R perpendicular to the auxiliary signal A. Equivalently, theresidue comprises the component of the main signal M that isperpendicular to the auxiliary signal A.

Referring now to FIG. 2, there is shown one embodiment of a 3-pulseadaptive MTI in accordance with the present invention. Given threeconsecutive pulses A, B, C, with adjacent pulses separated by the radarinterpulse period, the purpose of this design is to provide theGram-Schmidt decorrelation (A⊥B)⊥(C⊥B) with a minimum number of adaptivecancellers, i.e., N-1, or in this case, two cancellers. The circuitincludes a first input pulse line 110, a first delay means 112 forproviding an interpulse delay T to pulses on the input line 110, and afirst adaptive canceller 114 with a main input M and an auxiliary inputA and an output line. The first adaptive canceller 114 operates todecorrelate a signal applied to the main input M from a signal appliedto the auxiliary input and to generate a residue signal on its outputline 116. A switching means 118 with a plurality of switch positions isconnected for applying a pulse from before the first delay means 112 anda pulse from after the first delay 112 to the auxiliary and main inputs,respectively, of the first canceller 114, at one switch postion, and tothe main auxiliary inputs, respectively, of the first canceller 114, onthe next consecutive switch position. Thus, for the three consecutivepulses A, B, C, the first canceller 114 generates on its output lineconsecutively A⊥B and then C⊥B. A second delay means 132 is included forproviding an interpulse delay T to pulses on the output line 116 of thefirst canceller 114. A second adaptive canceller 134 with a main inputM, an auxiliary input A, and an output line 136 is provided fordecorrelating a signal applied to the main input M from a signal appliedto the auxillary input A and then generating a residue signal on theoutput line 136. The auxiliary input A is directly connected to theoutput line 116 of the first canceller 114. The main input of thecanceller 134 is connected to take delayed pulses from the output line133 of the second delay means 132.

The two delay means 112 and 132 may be implemented by any standard delaydevice which is capable of providing a delay equal to the radarinterpulse period. By way of example, such delay means may compriseshift registers, or standard delay line elements.

The first and second cancellers 114 and 134 may be implemented by avariety of analog or digital cancellers. By way of example, an analogclosed loop canceller, such as the Howells-Applebaum canceller, or thedigital open loop cancellers disclosed by Lewis and Kretschmer, Jr. inU.S. Pat. No. 4,086,592, may be utilized.

The switching means 118 may take a variety of configurations in thepresent design. In the embodiment shown in FIG. 2, the switching meanscomprises a commutating switch 120 and a commutating switch 126. By wayof example, these switches may be implemented by single poledouble-throw switches that throw between pulse echo returns. Thecommutating switch 120 has its pole connected via line 121 to the inputline 110 and includes three switch positions: position 119, a dummyterminal; position 122 connected to the auxiliary input of the canceller114; and position 124 connected to the main input of the canceller 114.The commutating switch 126 has its pole is connected to the output line123 from the first delay means 112 and has three switch positions 127,128, and 130. Switch position 127 is a dummy position. Switch position128 is connected to the auxiliary input of the canceller 114. Switchposition 130 is connected to the main input of the canceller 114. Theseswitches are designed to throw or commutate on every pulse transmission.

It should be noted that the switch position shown in FIG. 2 comprisesonly one of many switch configurations which could be utilized toimplement the switching means 118. By way of example, it is possible toreverse the switch configuration such that the pole of the switch 120 isconnected directly to the auxililary input of the canceller 114 and hastwo switch positions connected to the lines 121 and 123, respectively.Likewise, the pole of the switch 126 could be connected directly to themain terminal of the canceller 114 and could include two switchpositions connected to the lines 121 and 123, respectively.

In FIG. 2, the pulse signals A, B, and C are shown as being applied tothe line 110 at the times t₁, t₂, and t₃, respectively. The timedifference between any two adjacent times is the radar interpulse delayperiod T. It can be seen that the first pulse A is available on the line110 at time t₁. One interpulse delay period T later, the second echopulse B arrives at the line 110 and the first echo pulse A is nowavailable on the output line 123 for the first delay means 112. At thistime t₂, the switches 120 and 126 are in the positions labelled t₂ inorder to provide the B pulse echo to the auxiliary input of thecanceller 114 via the switch position 122 and to provide the A pulseecho to the main input of the canceller 114 via the switch position 130.Accordingly, the output from the canceller 114 at the time t₂ is Adecorrelated from B, i.e., A⊥B. The output A⊥B is applied to the secondinterpulse delay means 132. One interpulse period T later, i.e., at thetime t₃, the pulse echo C is now on line 110 and the pulse echo B isavailable on line 123. Likewise, because the switches 120 and 126 throwor commutate on every pulse transmission, the switches 120 and 126 arenow in their solid arrow t₃ positions, such that the echo pulse C isapplied to the main terminal of the canceller 114 via the switchposition 124, while the echo pulse B is applied to the auxiliary inputof the canceller 114 via the switch position 128. Accordingly, at timet₃, the output line 116 from the canceller 114 provides pulse Cdecorrelated from pulse B, i.e., C⊥B. This output C⊥B is applieddirectly via the line 116 to the auxiliary input of the canceller 134.The previous output on line 116 at time t₂, i.e., A⊥B, is now availableon the output line 133 of the second delay means 132. Accordingly, theoutput A⊥B is applied via the line 133 to the main input of thecanceller 134. The output of the canceller 134 on line 136 is then(A⊥B)⊥(C⊥B) at the time t₃. Note that this output is identical to thatof prior art Gram-Schmidt adaptive MTI cancellers but requires only twoadaptive cancellers instead of the three adaptive cancellers requiredpreviously. It should be noted that the above described output on line136, in essence, is the echo pulse A decorrelated from the echo pulse Band from that part of the echo pulse C which is decorrelated from theecho pulse B, i.e., all new information.

It should be noted that in the embodiment of FIG. 2, 3-pulse sequencesare used in the processing. Because an initial pulse time t₁ is used ineach sequence to read pulse samples on to the line 110 for the circuit,the dummy switch positions 121 and 127 are included on the switches 120and 127, respectively, in order to provide for proper 3-pulsesynchronization for these switches. Note that there are switchingconfigurations where dummy switch positions may not be necessary.

From the above, it can be seen that optimum clutter cancellation hasbeen obtained for an N pulse adaptive MTI while employing only N-1cancellers. This scheme has been accomplished with only the added costof a pair of switches, thus significantly reducing the cost of thedevice while maintaining optimum performance.

Referring now to FIG. 3, there is shown an embodiment of the presentinvention extended to higher order canceller configurations. The precisecanceller configuration shown in FIG. 3 is a four-pulse canceller.However, some of the circuitry for a 5-pulse canceller is also includedin this figure to illustrate of the higher order extension of thepresent invention. In this configuration, it should be noted that onlyone canceller is required per horizontal Gram-Schmidt level ofcancellers. In this context, a horizontal level of cancellers is of thetype shown in FIG. 1 wherein there are four separate horizontal levelsof cancellers to implement the 5-pulse MTI structure. Accordingly, thepresent invention for a N pulse adaptive MTI requires only N-1cancellers rather than N(N-1)/2 cancellers, as in the prior art.

Referring now to FIG. 3, the system therein comprises a plurality ofseries-connected cancellers. The first canceller in the series is a2-pulse canceller and includes a first input pulse line 150 forproviding echo pulses or partially decorrelated echo pulses, a firstdelay means 152, and a first adaptive canceller 154. The first delaymeans 152 provides an interpulse delay T to the pulses on line 150. Thefirst adaptive canceller 154 has a main and an auxiliary inputs and anoutput line 156. This canceller 154 again operates in the same manner asthe cancellers of FIG. 2 to decorrelate a signal applied to the maininput from a signal applied to the auxiliary input and to generate aresidue signal on the output line 156. In this configuration, pulses onthe first input pulse line 150 are applied directly to the first delaymeans 152 and to the auxiliary input of the canceller 154. The output online 158 from the first delay means 152 is applied to the main input ofthe canceller 154.

For each additional pulse added to the pulse canceller, there will be aseparate canceller circuit connected in series thereto. Accordingly, foran N-pulse canceller, there will be N-2 series connected cancellercircuits, with each canceller circuit associated with a different numberof pulses, where 1≦n≦N-2. Each of these canceller circuits includes anadaptive canceller with a main and an auxiliary input and an outputline, a plurality of tapped delay lines comprising serially connecteddelay means for providing a series of interpulse delays T, andcommutating switch means with a plurality of switch positions connectedfor applying pulses from taps on the delay line to the main andauxiliary inputs of the adaptive canceller. This commutating switchmeans operates to provide appropriate sets of two pulses from the tappeddelay line to the main and auxiliary inputs to effect one cancellerlevel of Gram-Schmidt pulse decorrelation on the output line of thecanceller. Note that the last of these canceller circuits has its outputline connected to the first input pulse line 150. The ultimate effect ofthis circuit operation is to provide the optimum Gram-Schmidtdecorrelation on a set of consecutive pulses.

For a 3-pulse canceller, there will be N-2 or one additional cancellercircuit. In FIG. 3 this 3-pulse canceller is shown as including an inputline 160, a tapped delay line 161 including delay means 162 with tapterminals S1 and S2 on either side thereof, an adaptive canceller 164,and commutating switch means 166. The purpose of the tapped delay line161 and the commutating switch means 166 is to provide the appropriateadjacent pair of pulses to the main and the auxiliary inputs of thecanceller 164. In the particular configuration shown in FIG. 3, thecommutating switch means 166 is implemented by two single-polefour-position switches 168 and 170. The switch 168 is connected at itspole to the auxiliary input of the canceller 164, while the pole of theswitch 170 is connected to the main input of the canceller 164. Itshould be noted that this 3-pulse canceller configuration, including thetwo delay means 162 and 52, the two cancellers 164 and 154, and the twoswitches 168 and 170, is almost identical to the configuration shown inFIG. 2. However, note that the commutating switch means 166 is slightlydifferent in that the pole positions have been reversed so that the polefor the switch 168 is now connected to the auxiliary input of thecanceller 164 while the pole for the switch 170 is now connected to themain input for the canceller 164. Also, there is an additional dummyterminal which will be discussed later.

A 4-pulse canceller is shown as including an additional cancellercircuit over that of the 3-pulse canceller. This additional cancellercircuit includes an input line 178 and a tapped delay line 179 with twoserially-connected delay means 180 and 182 for providing interpulsedelays T to pulses propagating thereto. Tap terminals S3, S4, and S5 areprovided in front of each delay means and after the last delay means.This additional canceller circuit includes a commutating switch means184 and an adaptive canceller 186 with a main input, an auxiliary input,and an output line 187. Again, the commutating switch means 184 maycomprise two single pole multi-position switches 188 and 190 forproviding pairs of echo pulse samples to the auxiliary and the maininputs of the canceller 186. The output line 187 of the canceller 186 isapplied to the input line 160 for the 3-pulse canceller.

For purposes of explanation of the 4-pulse canceller, the input echopulses applied on the input pulse line 178 are numbered consecutively as1, 2, 3, and 4, and occur at the times t₀, t₁, t₂, and t₃, respectively.

In the embodiment shown in FIG. 3, each of the switches 188 and 190 isshown as including four switch positions, one for each of the pulsesapplied onto line 178. These four switch positions ensure that theindividual switches will commutate in proper synchronism such that aftera 4-pulse sequence, the switch will be at its first position again. Inthis regard, the switch 188 has a dummy terminal D which will beconnected to the pole of the switch at time t₀, a position S3 which isconnected to the terminal S3 on delay line 179 and is connected to thepole of the switch at time t₁, a switch position S4 which is connectedto the terminal S4 in the delay line 179 and is connected to the pole ofthe switch at time t₂, and finally a switch position S5 which isconnected to the terminal S5 in the delay line 179 and is connected tothe pole of the switch at time t₃. Likewise, the switch 190 has a dummyswitch position D which is connected to the pole of the switch at timet₀, a switch position S4 which is connected to the terminal S4 in thedelay line 179 and is connected to the pole of the switch at time t₁, aswitch position S3 which is connected to the terminal S3 in the delayline 179 and is connected to the pole of the switch at time t₂, andfinally the switch position S3 which is connected to the terminal S3 inthe delay line 179 and is connected to the pole of the switch at timet₃. These switch configurations with the timing synchronism noted resultin the following outputs on the output line 187 for the canceller 186:at time t₁ the output 1⊥2=A; at time t₂ the output 3⊥2=B; and at time t₃the output 4⊥2=C.

These outputs A, B, and C are applied via the output line 187 to theinput line 160 for the 3-pulse canceller. These outputs are applieddirectly and by way of the delay means 162 to the auxiliary and maininputs of the canceller 164. Again, the commutating switch means 166determines which pair of pulses are applied to the auxiliary and maininputs of the canceller 164. In this regard, the switch 168 has fourswitch positions D, D, S1, and S2. The first switch position D is adummy switch position and is connected to the pole of the switch at timet₀. The second switch position D is again a dummy switch position and isconnected to the pole of the switch time t₁. The switch position S1 isconnected to the terminal S1 in the delay line 161 and is connected tothe pole of the switch 168 at time t₂. The switch position S2 isconnected to the terminal S2 in the delay line 161 and is connected tothe pole of the switch 168 at time t₃. Likewise, the switch 170 has fourswitch positions D, D, S2, and S1. The first switch position D is adummy switch position and is connected to the pole of the switch 170 attime t₀. The second switch position D is again a dummy position and isconnected to the pole of the switch at t₁. The third switch position S2is connected to the terminal S2 in the delay line 161 and is connectedto the pole of the switch 170 at time t₂. The fourth switch position S₁is connected to the terminal S1 on the delay line 161 and is connectedto the pole of the switch 170 at the time t₃. As noted above, the poleof the switch 170 is connected to the main input of the canceller 164while the pole of the switch 168 is connected to the auxiliary input ofthe canceller 164.

With the above described switch configuration and utilizing the timingdescribed, the following outputs are obtained on the input line 150 forthe canceller 164: at time t₂ the output A⊥B; at the time t₃ the outputC⊥B.

Accordingly, it can be seen that at time t₃ the output C⊥B will beavailable at the auxiliary input to the canceller 154 via the line 150,while the output A⊥B will be available at the main input of thecanceller 154 via the delay means 152 and the output line 158. Thus, attime t₃ the canceller 154 will provide the output (A⊥B)⊥(C⊥B). Thisoutput is the optimum Gram-Schmidt output, as described previously.

From the above description, it is clear how the present invention can beextended to higher order canceller configurations of 5-pulsecancellation and above. Each additional pulse would include its owncanceller circuit level. Each such canceller circuit level would includean nth input pulse line, wherein for that level n would be equal to N-2,an nth adaptive canceller, n series connected delay means for providinga series of interpulse delays T to pulses propagating on the nth inputpulse line, and with n+1 tapped terminals for taking pulse outputs infront of each of the end delay means and after the last of the n delaymeans, and nth commutating means with a plurality of switch positionsconnected for applying pulses from the n+1 terminals of the delay meansto the main and the auxiliary inputs of the nth adaptive canceller. Thiscommutating switch means operates to provide appropriate sets of twopulses from the n+1 terminals of the series connected delay means to themain and auxiliary inputs to effect the n+2 canceller level ofGram-Schmidt pulse decorrelation on the output line of the nthcanceller.

By way of example, for a 5-pulse canceller, there is shown in FIG. 3 aninput pulse line 200, n, where n is equal to N-2 or 3 series connecteddelay means 202, 204, 206, with n+1 terminals S6, S7, S8, and S9 takenfrom in front of each of the n delay means 202-206 and after the last ofthe n delay means; an nth adaptive canceller 208, and nth commutatingswitch means 210. Again, the commutating means may be comprised of twosingle-pole multi-position switches 212 and 214. Each of thesemulti-positions switches 212 and 214 includes five switch positions forthe five pulses utilized in the 5-pulse canceller. The pole for theswitch 212 is connected to the auxiliary input of the canceller 208,while the pole of the switch 214 is connected to the main input of thecanceller 208. The output of the canceller 208 is applied via the dashline 211 to the input line 178 for the 4-pulse canceller. For ease ofillustration, the switch positions for the switches 212 and 214 are notshown. It should also be noted that some adjustment will be required tothe switches 188 and 190 in the 4-pulse canceller level and the switches168 and 170 in the 3-pulse canceller level. For example, at least oneadditional dummy terminal must be added to those switches in order toobtain the proper synchronization of these switches for a 5-pulsecanceller.

It should also be noted that although the switch configurations shown inFIG. 3 utilize single pole switches with the poles connected to eitherthe auxiliary or the main inputs for the respective cancellers, avariety of other switch configuration could also be used to implementthe present invention. By way of example, additional switches could beutilized at each canceller level and the poles for those switches couldbe connected directly to the terminals of the appropriate delay line.

The present invention has been disclosed in the context of a commutatingswitch configuration, wherein a pulse is optimally decorrelated in anN-pulse canceller after N pulses. Then a new set of N pulses would beused in order to optimally decorrelate a second pulse. For example, in a3-pulse canceller the pulses A, B, and C would be required in order tooptimally decorrelate a pulse in this configuration. Then threeadditionally pulses D, E, and F would be required in order to optimallydecorrelate the next pulse. Thus, the output on line 136 in FIG. 2 wouldbe read out on every third pulse. This type of configuration isadvantageous in that it facilitates the switching to a new frequencyevery N pulses in order to avoid jammers. This configuration alsofacilitates the change of the pulse repetition frequency every N pulses.It should noted however, that the present invention could be configuredin order to provide optimally decorrelated pulses on every pulse, orafter a given number of pulses less than N pulses, after the initialpulses have been loaded into the circuit.

It should be noted that the present invention adaptive MTI and itsassociated components may be implemented in either analog or digitalform.

In the present invention, only one canceller is needed per horizontallevel of cancellers in the standard Gram-Schmidt configuration. Thus,for an N pulse adaptive MTI, only N-1 cancellers are required, ratherthan N(N-1)/2 cancellers as in the prior art. This reduction in thenumber of cancellers in the system significantly reduces its complexityand its expense while providing optimum clutter cancellation.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. An adaptive N pulse MTI processor for obtainingoptimum clutter cancellation with a minimum of hardware, where N is aninteger greater than 2, comprising:a first input pulse line; first delaymeans for providing an interpulse delay T to pulses on said first inputline; a first adaptive canceller with a main and auxiliary inputs and anoutput line, with said auxiliary input connected directly to take pulsesfrom said first input line, and with said main input connected to takedelayed pulses from said first delay means, said first adaptivecanceller for decorrelating a signal applied to said main input from asignal applied to said auxiliary input and generating a residue signalon said output line; N-2 series connected canceller circuits, with eachcanceller circuit associated with a different number of pulses, with thenth canceller circuit, where 1≦n≦N-2, including an nth adaptivecanceller, with a main and auxiliary inputs, and an output line, fordecorrelating a signal applied to said main input from a signal appliedto said auxiliary input and generating a residue signal on said outputline; an nth input pulse line; n series connected delay means forproviding a series of interpulse delays T to pulses propagating on saidnth input pulse line, and with n+1 terminals for taking pulse outputs infront of each of said n delay means and after the last of said n delaymeans; nth commutating switch means with a plurality of switch positionsconnected for applying pulses from said n+1 terminals to said main andauxiliary inputs of said nth adaptive canceller, said nth commutatingswitch means operating to provide appropriate sets of two pulses fromsaid n+1 terminals to said main and auxiliary inputs to effect the n+2canceller level of Gram-Schmidt pulse decorrelation on the output lineof said nth canceller; wherein the output line for the nth adaptivecanceller is connected to the n-1 input line for the n-1 cancellercircuit, and wherein the n=1 canceller circuit has the output line fromits adaptive canceller connected to said first input line, and whereinthe output line for said first adaptive canceller provides the fullGram-Schmidt pulse decorrelation output.
 2. An adaptive N-pulse MTIprocessor as defined in claim 1, wherein said commutating switch meanscomprises:a first commutating switch connected to said main input ofsaid nth canceller; and a second commutating switch connected to saidauxiliary input of said nth canceller.
 3. An adaptive N-pulse MTIprocessor as defined in claim 1, wherein said first and secondcommutating switches each have N switch positions.
 4. An adaptivemulti-pulse MTI processor for obtaining optimum clutter cancellationwith a minimum of hardware, comprising:an input pulse line; a firstdelay means for providing an interpulse delay T to pulses on said inputline; a first adaptive canceller with a main input and an auxiliaryinput, and an output line, for decorrelating a signal applied to saidmain input from a signal applied to said auxiliary input and generatinga residue signal on said output line; switching means with a pluralityof switch positions connected for applying a pulse from before saidfirst delay means and a pulse from after said first delay means to saidauxiliary and main inputs, respectively, of said first canceller on oneswitch position, and to said main and auxiliary inputs, respectively, ofsaid first canceller on the next consecutive switch position, such thatfor three consecutive pulses A, B, and C, said first canceller generateson its output line consecutively A⊥B and then C⊥B; second delay meansfor providing an interpulse delay T to pulses on said first cancelleroutput line; a second adaptive canceller with a main input, an auxiliaryinput, and an output line, for decorrelating a signal applied to saidmain input from a signal applied to said auxiliary input and generatinga residue signal on said output line, wherein said auxiliary input isdirectly connected to said first canceller output line and said maininput is connected to take delayed pulses from said second delay means,wherein the pulses on said second canceller output line aredecorrelated.
 5. A method for decorrelating a pulse in a multi-pulse MTIwith a minimum of hardware, comprising the steps of:obtaining threeconsecutive pulses A, B, and C, separated from each other by the MTIinterpulse period T; applying pulses A and B simultaneouly to the mainand auxiliary inputs, respectively, of a single first adaptivecanceller; decorrelating pulse A from pulse B to yield A⊥B; applyingpulses C and B simultaneously to said main and auxiliary inputs of saidsingle first adaptive canceller one interpulse period later;decorrelating C from B to yield C⊥B; applying A⊥B and C⊥B simultaneoulyto the main and auxiliary inputs, respectively, of a second singleadaptive canceller; and decorrelating A⊥B from C⊥B to yield (A⊥B)⊥(C⊥B).